It is known to be desirable in telecommunications systems to provide positioning methods for determining the geographical location of users within the system. This is useful for many reasons, such as emergency location, area-based billing, fleet management for trucking companies and other similar location based value-added services.
One way of mobile phone positioning is by means of a triangulation system, in which the location of a particular mobile phone is calculated using control signals from at least the three base stations closest to it. This system uses the assumption that the distance of the phone from a base station is proportional to the strength of the signal which the base station receives from it, or the time taken for the signal to travel between the phone and the respective base station. Thus the position of the phone can be determined by comparing the relative strengths or travel times of received signals between the three base stations and thus assessing the distance of the user from each base station. The actual location of the user is then obtainable geometrically since the location of the base stations is known and fixed.
There are a number of different methods of making the measurements for performing the above calculation, depending on the mobile system. In a Code Division Multiple Access (CDMA) system, the method which provides the greatest accuracy as well as having other advantages is a time-based method. This method is a downlink method in which a user's mobile phone measures the differences in the time of arrival (TOA) of signals from surrounding base stations in order to determine the relative distances between the user and each base station.
One particular problem which a time-based method is capable of overcoming is that of “hearability”. This problem occurs when the user's mobile phone is much closer to one of the base stations being used for positioning purposes than other surrounding base stations. In this situation, the signal between the user and the close station is so strong, that it is difficult to transmit and receive signals between the user and the other surrounding base stations. This problem occurs particularly in a CDMA system, because all base stations typically transmit at the same carrier frequency.
In the proposed wideband CDMA system (W-CDMA), which is suggested for the universal mobile telecommunications system (UMTS) standard there are proposed to be three types of control channel. The first type is broadcast control channels (BCH) which are to be used, for example, for transmitting frequency and frame synchronisation information. The second type is common control channels (CCCH) which are to be used for network access, for example paging services. These first two types are common to all mobile users. The third type is dedicated control channels which are to be allocated to individual users. These include channels used for handover and user registration. It is likely that measurements for the purpose of mobile phone location will use an existing BCH. Signals are transmitted in blocks called frames, transmitted over each channel. The order of transmission of frames from the different data streams is suitably selected for greatest efficiency. Since this is not a time division system, the problem of hearability can in principle be overcome by transmitting at reduced power or ceasing transmission from the closest base station for short periods of time. Such a time period is known as an idle period. It allows the remaining base stations being used for positioning purposes to communicate with the mobile phone, thus providing the ability to locate the user. This should not affect the transmission quality significantly since during such a time period, a user's mobile phone will often receive signals from its other nearby base stations if the mobile phone is in a soft handover (macrodiversity) situation.
In order to allow positioning to occur during idle periods it would be desirable to allocate idle periods such that nearby base stations do not have idle periods at the same time. This is so that during an idle period, a mobile phone will still be able to pick up signals from other nearby base stations and therefore the positioning of the mobile can continue using signals from other base stations.